1. Field of the Invention
The present invention is a chemical blending system. In particular, the present invention is a computer-controlled system for blending batches of concentrated chemicals from two or more chemical components for subsequent use in semiconductor fabrication facilities.
2. Description of the Related Art
Chemical generation or blending systems are used in a variety of industrial applications to blend two or more components or constituents to a desired concentration. In semiconductor fabrication facilities, for example, concentrated chemicals (which are usually provided by commercial chemical suppliers in solution with water) are mixed or diluted with DI (deionized) water before being sprayed on or otherwise applied to semiconductor wafers. Table 1 below lists a number of chemicals used in semiconductor fabrication facilities, and the concentration (in weight %) in which these chemicals are typically provided by suppliers.
TABLE 1 ______________________________________ Percentage Concentrate Chemical Symbol in Water ______________________________________ Hydrofluoric Acid HF 49% Acetic Acid HAC 99.7% Nitric Acid HNO.sub.3 71% Phosphoric Acid H.sub.3 PO.sub.4 80% Potassium Hydroxide KOH 30% Tetramethyl Ammonium TMAH 25% Hydroxide Hydrochloric Acid HCl 37% HF and Ammonium BOEs -- Fluoride Mixtures Ammonium Hydroxide NH.sub.4 OH 28-30% Sulfuric Acid H.sub.2 SO.sub.4 93-98% ______________________________________
When used in semiconductor fabrication facilities, the concentrated chemicals described above are commonly diluted with DI water (i.e., a diluent) to desired concentrations or assays. Concentrations in these applications are typically described in terms of weight % (weight percent) of concentrated or pure chemical in water. Hydrofluoric Acid (HF), for example, is often diluted with high purity water to concentrations ranging from about 0.5%-5% HF by weight when used for etching and cleaning processes. Tetramethyl Ammonium Hydroxide (TMAH) is often diluted to about 2.38 weight % for use as a positive photoresist developer. Non-aqueous blended chemicals, and blended chemicals with three or more components, can also be generated.
Chemical blending systems blend the chemicals to a desired concentration which is sometimes known as the nominal or qualification concentration. A high degree of accuracy is also required. The range or window of acceptable concentrations surrounding the qualification concentration is known as the qualification range, and can be defined as a weight % error with respect to the qualification concentration, or by upper and lower qualification range concentrations.
A known chemical blending system which is commercially available from FSI International of Chaska, Minn., the assignee of the present invention, is disclosed generally in commonly assigned application Ser. No. 08/355,671, filed Dec. 14, 1994 and entitled "Apparatus For Blending Chemical And Diluent Liquids". This chemical blending system includes a mix tank for the blended chemical, a recirculation line having an inlet and outlet in the mix tank, and a pump in the recirculation line. A source of a first constituent of the blended chemical, such as DI water which is used as a diluent, is coupled to the mix tank through an inlet and supply line. A source of a second constituent of the blended chemical, such as the concentrated chemical to be diluted, is coupled to the recirculation line through an inlet, source line and adding valve. The adding valve is located in the recirculation line on the suction side of the pump (i.e., between the pump and the inlet of the recirculation line) , and is controlled by a microprocessor-based control system. When the pump is operating and the adding valve is open, concentrated chemical is drawn into the recirculation line. Recirculation of the blended chemical through the recirculation line causes the blended chemical and added concentrate to be thoroughly mixed.
Concentration of the blended chemical is monitored by conductivity-type sensors in the recirculation line between the pump and inlet. The sensors are coupled to the control system through analyzers that convert the conductivity readings provided by the sensors to concentration values used by the control system.
The control system initiates a chemical blending cycle by filling the mix tank with a desired quantity of DI water and activating the pump to recirculate the blended chemical within the tank and recirculation line. The concentrate adding valve is then opened to provide a continuous flow of concentrated chemical into the recirculation line. During this continuous injection phase of the blending cycle the concentration of the blended chemical is monitored continuously and compared to a coarse blend setpoint. The coarse blend setpoint can be empirically determined, and represents a concentration which is sufficiently less than the qualification concentration that the continuous addition of concentrated chemical will approach, but not exceed or overshoot, the qualification concentration if the addition of concentrated chemical is stopped when the measured concentration has increased to the coarse blend setpoint. Once the control system determines that the measured blended chemical concentration has reached the coarse blend setpoint, it closes the concentrate adding valve.
The control system then periodically opens and closes the concentrate adding valve during a periodic injection phase. Relatively small quantities of the concentrated chemical are added during the time periods that the valve is open, and the added concentrated chemical is mixed with the blended chemical while the valve is closed. The concentration of the blended chemical is continuously measured and compared to the qualification concentration during this periodic injection phase. To ensure that the concentration measurements are made in homogeneous and thoroughly blended chemical, the duty cycle of the period during which the concentrate adding valve is open is relatively short compared to the duty cycle of the time period during which the valve is closed. Furthermore, to minimize the chances that the concentration will exceed the qualification range, the duty cycle of the time period during which the valve is open is relatively short so as to increase the concentration in relatively small increments. In one embodiment, for example, the open valve duty cycle is about six seconds while the closed valve duty cycle is about twenty-four seconds. When the measured concentration reaches the qualification concentration, the control system qualifies the blended chemical batch and ceases further concentrated chemical addition. The blended chemical can then be pumped to its point of use.
The conductivity-type sensors used in the chemical blending system described above are capable of providing continuous and almost instantaneous measurements of the blended chemical concentration. The accuracy of the measurements provided by the conductivity-type sensors is also good. Nonetheless, blended chemical concentration variations within the range of accuracy that can be provided through the use of conductivity-type sensors can result in semiconductor fabrication process variations. These process variations can detrimentally affect the physical and electrical characteristics of the semiconductor wafers being processed. The problems associated with these process variations will become even more critical as the circuit geometries on the wafers become smaller and the circuit patterns more complex. Chemical blending systems capable of blending chemicals to higher concentration accuracy levels or tolerances are therefore needed to keep pace with other advances in semiconductor fabrication processes.
The use of titration analyzers to measure the concentration of blended chemicals produced by chemical blending systems is also known. Titration analyzers are commercially available from a number of suppliers including Applikon Dependable Instruments of the Netherlands, through its North American distributer Applikon Analyzers, Inc. of Kingwood Tex. When actuated, analyzers of this type draw a sample of the blended chemical. The sample is then titrated with reagents and its pH or pH inflection point measured to determine the concentration of the blended chemical. Titration analyzers are capable of providing concentration measurements to a higher degree of accuracy than conductivity-type sensors (e.g., to less than about 0.10% relative error (i.e., error/setpoint) at three standard deviations or three sigma (3.sigma.)).
For a number of reasons including the minimization of storage container space, the propensity of containers to contaminate chemicals during prolonged storage and the tendency of concentration values to change with time, chemicals are typically blended relatively frequently and in relatively small batches. The batches of blended chemical are then used relatively soon after they are produced. Chemical blending systems must therefore be capable of quickly blending the batches of chemical to the desired concentration. Unfortunately, titration analyzers have a relatively slow measurement response time (about 3-5 minutes per measurement) compared to the nearly instantaneous response of conductivity-type sensors. Since a number of concentration measurements are typically required before a batch of blended chemical can be qualified, the use of titration analyzers can increase the length of time required to blend a batch of chemical.
It is evident that there is a need for improved chemical blending systems. In particular, there is a need for chemical blending systems capable of quickly blending batches of chemical to a very high degree of accuracy. To be commercially viable, the chemical blending system must also be highly reliable.